1. Field of the Invention
The present invention relates to a method and an apparatus for measuring a distance and, more particularly, for measuring the external shape of a three-dimensional object.
2. Description of the Related Art
According to one of the distance measuring methods known heretofore, positions on the surface of a three-dimensional object are measured by scanning the surface with a beam of slit light while rotating a scanning mirror, and detecting the points in time when the slit light reflected from the object surface passes through sensing cells arranged on an image pickup plane.
FIG. 11 shows an exemplary conventional apparatus which carries out such distance measuring method. In this example, an infrared or similar laser beam emitted from a laser light source 1 is formed into slit light via an optical device 2. The slit laser light is then reflected by a scanning mirror 3 consisting of a galvano mirror or the like disposed at a predetermined position. The slit light 3S thus reflected from the scanning mirror 3 scans the surface of a three-dimensional object 4, which is to be measured, with rotation of the mirror 3.
The slit light reflected from the object 4 is focused on an image pickup plane 7 via an optical device 6. In a controller 5C comprising a differentiator, an integrator and other components, the rotational position of the scanning mirror 3 is detected by finding the point in time when the slit light reflected from the object 4 passes through each of the image sensing cells, i.e., light receiving cells P.sub.i,j arranged two-dimensionally on the image pickup plane 7. The distance up to the object 4 is measured trigonometrically per sensing cell P.sub.i,j on the image pickup plane 7 from the positional relationship among the rotational position of the scanning mirror 3, the image pickup plane 7 and the scanning mirror 3.
FIG. 12 schematically shows the principle of measuring three-dimensional coordinate positions on the surface of the object 4 according to the method of triangulation. The distance Z.sub.i,j from the image pickup plane 7 to one surface position on the object 4 whose reflected slit light is focused on the sensing cell P.sub.i,j of the image pickup plane 7 can be obtained trigonometrically from Eq. (1) by using the distance B.sub.i,j between the focused position on the image pickup plane 7 and the rotational center of the scanning mirror 3, the angle .alpha..sub.i,j formed by the image pickup plane 7 and the slit light reflected 3S from the mirror 3, the angle .beta..sub.i,j at which the slit light is incident upon the image pickup plane 7 from the object 4, and the vertical distance A from the rotational center of the scanning mirror 3 to an extension of the image pickup plane 7. ##EQU1##
The three parameters B.sub.i,j, .beta..sub.i,j and A in Eq. (1) are constants determined uniquely by the arrangement of the light source 1, the optical device 2, the scanning mirror 3, the optical device 6 and the image pickup plane 7. The angle .alpha..sub.i,j formed by the image pickup plane 7 and the slit light reflected 3S from the scanning mirror 3 can be obtained by, as mentioned, detecting the rotational position of the mirror 3 from the point in time when the slit light reflected from the object 4 passes through the sensing cell P.sub.i,j on the image pickup plane 7. Therefore it is possible to trigonometrically calculate the distance Z.sub.i,j from each cell on the image pickup plane 7 to the object 4.
However, the following problems exist in the conventional distance measuring method and apparatus mentioned above.
(1) Difficulties in deriving parameters
For accurately finding the distance Z.sub.i,j up to the object 4, it is necessary to derive the four parameters B.sub.i,j, .beta..sub.i,j, A and .alpha..sub.i,j with high precision. However, since the parameter .alpha..sub.i,j represents the angle formed by a plane parallel with the image pickup plane 7 and the slit light reflected from the scanning mirror 3, it is difficult to derive the parameter .alpha..sub.i,j with high precision even by accurately finding the point in time when the slit light reflected from the object 4 passes through the sensing cell P.sub.i,j on the image pickup plane 7. Extremely great difficulties are also unavoidable in measuring the parameters B.sub.i,j, .beta..sub.i,j and A with high precision.
(2) Difficulties in deriving distance from angle data
Even if the four parameters B.sub.i,j, .beta..sub.i,j, A and .alpha..sub.i,j can be obtained precisely, a long time is required for measuring one distance image because a considerably long time is needed to execute the calculation of Eq. (1) for every one of the sensing cells P.sub.i,j on the image pickup plane 7.